When wolves were reintroduced to Yellowstone National Park in the United States after being absent nearly 70 years, the most remarkable “trophic cascade” occurred. What is a trophic cascade and how exactly do wolves change rivers? George Monbiot explains in this movie remix.

Narration from TED: “For more wonder, rewild the world” by George Monbiot. Watch the full talk, here: http://bit.ly/N3m62h